Toughening with rubber-containing compounds has been applied to enhance the poor crack-propagation resistance in brittle epoxy thermosets. Incorporating a small amount (typically ≦15 wt %) of discontinuous rubber phase has been known for its effectiveness to toughen epoxy thermosets with glass transition temperature (Tg) in the range of 50° C.<Tg<130° C. This enhancement is attributed to the development of the process zone, consisting of plastic deformations, located in front of the crack tips. The development of the process zone, triggered by cavitated rubber particles, dissipates the strain energy prior to the crack initiation. As a result, the crack propagation under mechanical loading is impeded, and the tendency of material catastrophic failure is improved.
However, this rubber toughening technique does not always deliver an efficient enhancement in fracture toughness (KIC) in high Tg (Tg>150° C.) epoxy thermosets. It is known that the highly crosslinked network structures in those epoxy thermosets further restrict the chain movement so that their Tg values are high. The highly crosslinked network structures may also confine the propensity of process-zone development when the rubber toughening technique is applied. Therefore, the toughenability, ΔKIC (KIC toughend−KIC untoughened), and high Tg is one of the well-known dilemmas for the “rubber filled” epoxy thermosets.
Engineering thermoplastics such as polyethersulfones and polyetherimides have also been used for improving toughness of the epoxy thermosets. This method includes dissolving large amounts of thermoplastics into the epoxy resin, subsequently mixing a hardener into the mixture, and curing the composition at an elevated temperature. However, incorporation of the thermoplastic into epoxies often takes a significant amount of time. It also leads to a dramatic increase in viscosity and lower modulus in the epoxy thermoset. Therefore, it is desired to limit the usage of engineering thermoplastics that lead to insufficient toughness improvement in epoxy thermosets.
The objective of this invention is to provide a curable epoxy resin composition that possesses good toughness and heat resistance. This epoxy composition can be used in applications such as electrical laminates, epoxy molding compounds, coatings, and fiber reinforced polymer composites.